In the design and construction of nautical vessels, such as crew boats and work boats for both the off-shore drilling industry and the fishing industry, it is always an objective to deliver a boat exhibiting increase speed, efficiency, reduced fuel consumption, improved overall handling, and reduced skin friction of the water, especially in high seas. Several attempts have been made in the prior art to deal with one or more of these parameters. However, the prior art has not been able produce an exhaust system that accomplishes all of the objectives at the same time.
The prior art systems have generally utilized underwater exhaust of engine gas as a means of muffling the sound of a running engine. In fact, the prior art contains several examples of methods which have been suggested to reduce the sound of an engine and simultaneously attempt to increase the efficiency of the vessel. In some cases, the exhaust gases from the propulsion system have been directed into the propeller area in the aft section of the vessel However, this method has not met with great success because vessel efficiency is destroyed by cavitation which takes place at the propeller area as exhaust gas passes by the propeller.
The energy which is produced by a rotating propeller as it drives a vessel in the forward direction, must be uniform in nature in order for the vessel to properly navigate the open seas. When gases are introduced into the flow system of the propeller, irregular forces adversely effect the propeller action, causing a dramatic decrease in vessel performance and operating efficiency.
As a means of propelling a boat, the prior art has used a jet propulsion system where jets of fluid such as water or gas are expelled under the aft hull section of a boat in order to drive the vessel in a forward direction. Jet propulsion systems, which are generally inefficient in propelling a boat, should not be confused with the utilization of exhaust gas from an engine that is used to rotate a propeller in order to move the boat. In jet propulsion systems, fluid is expelled under the hull with great force: whereas in exhaust gas systems, exhaust gas is expelled at a moderate rate as it is produced by the engine that powers the boat.
U.S. Pat. Nos. 4,393,802 (Rizzo) and 4,002,136 (Michalak) are examples of prior art exhaust systems which are employed to facilitate forward motion of a vessel. The Rizzo patent relates to a system which uses a longitudinal channel and a step to define a recessed void into which exhaust gas is introduced. Thus, the Rizzo system employs an actual cavity in the hull below the vessel as part of its propulsion system. The Michalak patent teaches a system for placing the exhaust in the stern of a vessel. The placement of the exhaust system muffles noises produced by the ship's engines. However, because the system is located so far back near the stern section of the ship, only a small section of the bottom of the hull of the boat is available. Thus it is impossible to create any increase in lift or reduction in skin friction along the hull of the vessel.
There is thus a need for an underwater exhaust system which acts to increase the speed of a boat, reduce fuel consumption, improve handling, and reduce skin friction of water. The present invention is directed toward filling that need by providing an exhaust system that uses an exhaust box built into the hull of the vessel over the bottom of the boat and a diffuser position between the exhaust box and the boat bottom. Exhaust gas from an engine passes through the exhaust box where it is cooled by circulating water and then out through exhaust ports provided in the diffuser. Exhaust gas escaping from the diffuser is passed along the bottom of the hull, which has a stepped construction near the diffuser.